Photo-masks (also referred to as reticles) used in the manufacturing of integrated circuits are exposed during the manufacturing process to DUV (Deep Ultra-Violet) radiation and transfer the image patterned on the photo-mask, to the photo-sensitive material coated on a substrate (e.g., a Silicon wafer).
An image is typically transferred at a reduction ratio of 1:4, in which case all pattern dimensions are de-magnified 4 times on wafer level in order to obtain the desired dimensions on the final product (the wafer). Other reduction ratios can be used.
Line width size distribution across the photomask may be determined by CD (Critical Dimension) measurements, which measure CD directly by metrology tools or inspection tools, at typical accuracies of about 1 nm (nanometer) on wafer level.
It is well known in the semiconductors industry that a 1 nm variation in CD of transistors critical layers of front-end integrated circuit (IC), can result in up-to 100 MHz loss of processing speeds of the final product. Therefore, mapping of CD variations across the mask is an indispensable necessity for wafers yield analysis and performance of the final made IC.
At typical front end IC (integrated circuits) manufacturing processes with design rules of dense lines and spaces with CD of 45-90 nm a 1% transmission difference on photomask level, translates to more than 1 nm of CD variation on wafer level.
Inspection tools based on optical image processing are not able to detect accuracy values smaller than 1% of transmission differences, due to the small dynamic range of detection devices, such as CCD cameras (typically 8-10 bits), and the small field of view, which cannot ensure proper statistical averaging to be above noise level.
Therefore, accuracy of CD measurements is typically in the order of 1 nm.
Semi-direct measurements of CD are also carried out by optical metrology tools. Such tools for measuring CD distributions on photomasks are based upon scatterometry techniques, which analyze scattered light signals at variable angles or wavelengths, at two orthogonal polarizations.
Such techniques use simulation algorithms to convert measured results into CD data distributions. However, scatterometry requires predefined geometrical features on photomasks, which can be patterned only on scribe-lines, outside of the IC real active area. Measurement accuracy is also limited to about 1 nm on.
Additional methods available today to IC wafer fabs and photomask-shops include using a scanning electron microscope (SEM) and an atomic-force microscope (AFM). However, SEM method is also known to have accuracy levels of about 1 nm, and it is a destructive method (needs to measure in scribe-lines to avoid real pattern damage by electron-beam). Moreover, SEM is a slow process that limits the number of measurement points severely. Also, this technique requires the removal of the pellicle, which necessitates re-cleaning and re-pelliclizing and that actually makes the inspection itself redundant.
AFM method, though accurate enough, is so slow by nature (tens of minutes per point), so it can only be used for test and calibration, but not for mapping CD distributions.